In Internet Protocol (IP) systems, especially wireless IP systems such as High Speed Packet Access (HSPA) (i.e. enhanced uplink and High speed data packet access), the services must work on a wide range of operating conditions. The operating conditions depend on a number of factors:
The access type that is used, such as HSPA, Dedicated Channel (DCH) bearers, Enhanced Data Rates for GSM Evolution (EDGE), etc.
The channel conditions for the current user, i.e. if the user has good or bad channel conditions.
The current cell load in the system.
The traffic mixture, i.e. if all users are, for example, using Voice over IP (VoIP) or if there is a mixture of VoIP users, video telephony users and users doing web surfing or file transfer.
Specifically for HSPA, different schedulers give very different performance for different users.
System load. At high load situations routers drop packets as queues get full.
All these different operating conditions result in different performance problems and they require different adaptations in order to improve the quality of a VoIP session.
For circuit switched (CS) voice with Adaptive MultiRate (AMR), it is possible to adapt the speech codec (also referred to as source codec) bit rate and the channel coding bit rate so that:
For good channel conditions, an AMR mode with a high bit rate, for example AMR122, which allows for a quite small amount of channel coding, may be used. This gives the highest quality but makes it less resilient to channel errors.
For poor channel conditions, an AMR mode with a low bit rate, for example AMR475, which allows for extensive channel coding, may be used. This improves the resilience against channel errors while sacrificing some clean channel performance.
For channel conditions in-between these extremes, an AMR mode with medium bit rate, for example AMR74, which allows quite a lot of channel coding, may be used.
In CS systems such as GSM, W-CDMA the sum of speech coding bits and channel coding bits is constant. This is however not necessarily the case for PS systems. Further, for CS systems, adapting the bit rates of the source coding and channel coding therefore allows for maximizing the speech quality for good channel conditions and maximizing the resilience for poor channel conditions.
For IP systems, adapting the bit rate may or may not change the amount of channel coding, depending on the design of the IP system. For example:                Some IP systems may adapt the channel coding in a similar way as CS systems do.        Some IP systems may always add a fixed amount of channel coding, or a fixed modulation scheme. If, for example, the channel coding is fixed, for example a rate 1/2 channel code, then the size of the transmitted block is always proportional to the size of the data packet that is being transmitted.        Some IP systems may have a fixed channel coding but may allow for sending several packets in the same transmission block, if the channel conditions are good enough.        Some IP systems, such as HSPA, may adapt both the amount of channel coding and the number of IP packets that are packed into one transmission block.        
It should be clear that, for IP systems, adapting the source codec (such as selecting an AMR mode) and channel codec bit rates works for some operating scenarios and some operating conditions but not for all of them. Bit rate adaptation works not so well when the system is packet rate limited. There is also a general trend in the industry to move towards separating the source coding from the channel coding. In this case, reducing the bit rate, which gives smaller IP packets, but this does not necessarily mean that more channel coding (=error protection) is automatically applied.
VoIP also have to work for different combinations of access methods. One user can for example use HSPA while the other user in the session may for example use EDGE, Telecoms & Internet converged Services & Protocols for Advanced Networks (TISPAN), Generic Access network (GAN)/Wireless Local Area Network (WLAN) or a type of Digital Subscriber Line (xDSL) network. These access methods have different properties and one adaptation scheme that is designed (or optimized) for one specific access method may not work equally well for another access method.
To further complicate the problem, different systems have different capabilities, which enable adaption in different ways. Some examples of capabilities are:                The flexibility in modulation schemes and channel coding.        The smallest and largest possible transmission block sizes are different for different systems.        Some IP systems allow for transmitting several IP packets in one transmission block, other IP systems might not allow this.        
There may be other capabilities that are different for different systems.
An additional problem is that the VoIP application may not know what access type that it is being used. VoIP applications implemented in a cellular phone might be aware of the access type, but if the VoIP application is implemented in a PC (laptop), which uses the cellular phone is used as a modem, then the required signaling between the VoIP application and the cellular phone to exchange this information may not exist. To simplify the implementation, it may also be desired to separate the source coding from the transmission. In any case, the VoIP client will only know about its own access type. It is unlikely that signaling will be added to inform a first client of the access type that a second client is using.